i'd say it'd be engine oil leaking onto the exhaust because;
a) the rocker cover always leaks oil from the rear corners, the bolts come loose
b) when doing highway speeds in 4th or 5th the engine is bascially idling, which ,means that the viscous fan will barely be turning(ie not blowing heaps of air past the box area)
c) get an auto, they are cooler anyway.
give that area of the engine a good degrease, retension the bolts(or replace the gasket) and retest